[57] High-powered rifle
bullets travel faster than sound. Given a large enough propulsive
charge, bullets can penetrate the hypersonic range. Why not fashion
bullets resembling hypersonic models and fire them out of guns and
carefully watch them with instruments as they streak by? Although the
idea sounds a bit radical, it is quite sound-provided that a properly
instrumented firing range is available.

[58] The first Ames hypersonic tunnel, with a test section
of 10 x 14 inches. High-pressure air was supplied by the adjacent
12-foot pressure tunnel. Note the large battery of vacuum
pumps.

....Lewis. Not only did these wind tunnels
have much of the appropriate instrumentation, they could provide a
supersonic airstream for the bullets to fly into, thus extending
their ranges into the hypersonic realm. In other words, the
gun-launched models could be fired upstream to attain very high
relative velocities. To illustrate: a gun firing a model at 4000 feet
per second upstream in a tunnel operating at Mach 2 produces a
relative Mach number of 7; the combination of 8000 feet per second
and Mach 3 results in a relative Mach number of 15. Such high Mach
numbers in a conventional wind tunnel would normally call for
fantastically large expansion-ratio nozzles with the attendant danger
of air liquefaction. However, with the supersonic wind tunnel
actually operating at only Mach 2 or Mach 3, air liquefaction was no
concern at all. The relative Mach number was what counted
experimentally. Furthermore, the test Reynolds numbers would be
realistically high, as would the air temperatures at critical points
on the model. There was little doubt that a "counterflow" facility
would solve some of the design problems of hypersonic wind tunnels,
but what new problems would arise?

Shock waves appear as shadows trailing away from this
7-inch model. The model was fired from a 3-inch smoothbore naval gun
into still air at Mach 1. 6.

The concept of a counterflow tunnel was
proposed by H. Julian Allen at Ames in 1946, but his so- called
supersonic free-flight tunnel did not become a reality until late
1949. First, the problem of firing winged models from a gun with a
cylindrical barrel had to be solved. Special sabots or projectile
carriers were developed that enclosed the noncylindrical models while
they were inside the gun. The sabots peeled off as the
[59]
model left the muzzle heading upstream into the wind tunnel test
section. Inside the test section, the flight of the model was
"stopped" by a light flash about 1/3 microsecond long. Photos of the
bulletlike models streaking up the tunnel provided more insight into
hypersonic airflow than the experimenters had dared hope. The
high-temperature turbulent gases in the shock waves and boundary
layers cast clear-cut shadows on the screens. (These shadowgraphs are
not schlieren photos. They have the same origin as thermal shadows
cast by hot air rising from a home radiator on a bright winter day or
from a hot roadway in the summer.) In the Ames supersonic free-flight
tunnel the shadowgraphs revealed all the intricacies of hypersonic
airflow around various projecting portions of the model-a sort of
aerodynamic X-ray of tunnel airflow.